The geometries, chem. shifts, aromaticity, and reactivity of finite-length open-ended armchair single-walled carbon nanotubes (SWCNTs) have been studied within d. functional theory. The widely used model of capping hydrogen atoms at the open ends of a SWCNT changes the chem. activity of the SWCNT and destabilizes the frontier MOs. The edge π-orbital of the open ends enhances both π- and σ-aromaticity of the first belt of hexagons of carbon atoms at the open ends. The effect of the open ends on the structure and chem. reactivity of the SWCNT reaches only the first several layers of the hexagons of carbon atoms. Addns. of carbene and dichlorocarbene to the nanotube reveal that the open ends have higher reactivities than the inner regions. [on SciFinder(R)]